There are known various very small, solid state accelerometers which can be used in acceleration sensors. Applications for such sensors include missile arming devices and actuators for airbags in automobiles Other known accelerometers for use in actuating airbags in motor vehicles are relatively complex devices which depend upon the movement of balls within the cylinder which is affected by fluid pressure and traveling friction.
U.S. Pat. No. 4,598,585 issued to Boxenhor teaches a planar inertial sensor with a sheet member having one inner planar element adapted for angular motion. A base member supports the peripheral region of a sheet member in a plane perpendicular to the Z axis. The sheet member encloses a first pair of opposed C shaped void regions disposed symmetrically about the Y axis and a common point where the X, Y, and Z axes intersect. The portions of the sheet member between the opposed ends of the void regions are flexible The portion of the sheet member interior to the void regions has a mass imbalance across the Y axis, i e , one side has more mass than the other.
U.S. Pat. No. 4,483,194 issued to Rudolf teaches an integrated accelerometer having a flap fixed to a carrier by two beams which are disposed symmetrically and in line with one side of the flap. An electrode deposited on a plate permits acceleration to be measured by measuring the corresponding variation in capacitance between the flap and the electrode. The two symmetrical attachment means operate in a torsion mode. The accelerometer is a semiconductor with a semiconductor carrier substrate and a semiconductor flap member The flap member in the attachment means is formed of a single thin piece of monocrystaline silicon.
U.S. Pat. No. 3,498,138 issued to Stewart teaches an accelerometer with a sensing member having a nonconductive wafer divided, by a slot, into radially interior and exterior portions. The exterior portion is support for rotation relative to the interior portion by a pair of flexible hinge members. Translation of the outer portion results in restoring forces tending to produce only translational displacement thereof. A servo system positions the exterior portion to a neutral position and includes magnetic means for super-imposing a permanent magnet's unidirectional field with a field produced by an electromagnet, to apply a restoring torquing force couple to the sensing member. The working air gap of the permanent magnet circuit is varied to compensate for changes in the characteristic of the magnet due to temperature variations.
U.S. Pat. No. 3,478,604 issued to Evans teaches a tapered cantilever beam composed of P type silicon which is mounted to lie on a plane substantially at a right angle to the direction of motion of a body and receives stress or pressure at its tip end which is applied by a moveable ball weight. On each face side of the beam is an N type epitaxial resistor wired into a circuit configuration to form a unijunction transistor. The flexing of the beam serves to vary the resistors in an opposite sense and to control a transistor connected into a relaxation oscillator circuit.
Further, various silicon accelerometers are known wherein a pedestal suspends a surrounding sensor member through two opposing torsion bars. However, if the longitudinal stress along a torsion beam becomes compressive instead of tensile the behavior of the beam can change, allowing the beam to bend more under load and perhaps even buckle. Such a structure is disclosed in U.S. Pat. No. 4,736,629. It would be desirable to avoid a compressive force in the torsion beam and, indeed, to build in tensile stress These are some of the problems this invention overcomes.
Referring in particular to FIG. 4 of U.S. Pat. No. 4,736,629, the illustrated structure isolates the torsion arms from substrate/pedestal stresses, but it does not, by itself, create a tension in them. Thus, if the electroplating process produces beams in compression, they will remain in the state. Further, the structure is asymmetric and under acceleration upward out of the plane of the page (referring to FIG. 4), the ring structure 116, being held only at the left side through beam 114, will bend downward toward the right, tilting the plate 128 with respect to the substrate. This will reduce the linearity of the device, i.e., the linearity of the change in capacitance with acceleration. Under some circumstances one corner of the plate could deflect enough to touch the substrate. Tilting can also increase the sensitivity of the device to acceleration in the plane of the device perpendicular to the axis of the torsion arms (though both rotation of the plate around the torsion arms and symmetric bending of the torsion arms will also tend to increase off-axis sensitivity).
Referring to prior art FIG. 1, the beams, (10,11) supporting the deflection plate (12) of a torsion beam accelerometer (13) should not be under compressive stress, since this can cause the beams to deflect or even buckle under acceleration. Compressive stress can be introduced either in the manufacturing process, or as a result of differential thermal expansion between the supporting pedestal and the substrate on which it rests. For example, assume that as a result of temperature changes, the material comprising the substrate (14) shown in FIG. 1 expands relative to the material forming the pedestal (15), torsion beams (10,11) and deflection plate (12). The pedestal (15) (assumed thin in relation to the substrate), being bonded to the substrate (14), will expand relative to the deflection plate (13), thereby compressing the torsion beams (10,11).
If the expansion coefficient of the substrate material is less than that of the deflection plate structure material, at ambient temperatures above the bonding temperature the beams will be in tension, and for ambient temperatures below the bonding temperature, they will be in compression. At temperatures above the bonding temperature, the plate will expand relative to the pedestal, which is constrained from expanding freely by the lower thermal expansion coefficient material forming the substrate. Therefore, the beams will be Placed in tension. At temperatures below the bonding temperature, the opposite will be true. If the bonding is done at high temperature, then the beams will always be in compression. This will be the case, for instance, in an embodiment where a patterned, boron-doped silicon wafer is anodically bonded to a glass wafer at 400.degree. C., cooled, and then the undoped portion of the silicon etched away to leave behind the accelerometer structure (See L. Spangler and K. D. Wise, "A New Silicon-on-Glass Procedure for Integrated Sensors", IEEE Sensor and Actuator Work Shop, Hilton Head, pp 140-142, June 1988, the disclosure of which is incorporated herein by reference).